The present invention relates to a method of applying a wear-resistant coating to a strip of metallic carrier material, for example in the manufacture of scrapers, blades and the like.
The invention is of particular, but not exclusive utility in the manufacture of scrapers, blades and the like, particularly coating and smoothing scrapers for moving paper webs.
Developments in paper-coating technology tend towards coating plants with higher and higher production rates. Plants exist today with paper web speeds of up to 1500 m/min. and more, the width of the paper web sometimes being as much as 8 meters or more. Such plants demand great precision of the doctor used for such coating particularly since the quantity of coating composition applied is usually not moe than 5-25 g/m.sup.2 paper surface. In this field it is important that the layer of coating composition on the paper web must be perfectly uniform.
The coating result of such a coating plant is affected to a great extent by the length of the blade bevel in the direction of travel of the web in relation to the spring force applied on the flexible blade. The surface pressure applied, i.e. the pressure per unit surface area exerted via the bevel on the surface of the paper, known as the specific surface pressure, has decisive significance on the quantity of coating composition remaining on the web. A specific ratio therefore prevails between the extent of the bevel and the yielding properties of the blade. To prevent changes in the extent of the bevel surface due to wear during the coating process, therefore, such coating blades are now normally pre-ground to ensure a bevel extent and bevel angle which correspond as closely as possible to the actual conditions prevailing during the coating process itself. It is also important that the flexibility of the blade is adjusted to the prevailing spring force so that the blade will adjust to an uneveness in the web and also allow through defects such as lumps or thicker patchs in the paper web.
Conventional doctor blades give a good coating result, but they have the drawback of being subject to rapid and uneven wear and must therefore be replaced after only a small part of the blade material has become worn. This is because, for practical reasons, the coating blade, which generally cooperates with one side of the coated paper web and a rubber-clad support roller carrying the other side of the paper web, is wider than the paper web. Furthermore, pigment dispersions of clay in water are usually used for coating paper, which means that both the coating composition and the paper web itself have an extremely abrasive effect on the edge of the blade. On the other hand, those parts of the blades located beyond the paper web and thus merely in contact with the rubber-clad support roller during the coating process, are subjected to negligible wear. This means that the edge of the blade, initially straight, will become worn and somewhat concave along the sections where the paper web runs and after a while the coating across the web will become uneven. In practice, therefore, conventional blades must be replaced after only a few hours of use. This is expensive, not only from the material point of view, but also because it incurs expensive shut-downs followed by new running-in periods.
A great deal of work has been put into endeavours to increase the service life of conventional coating blades and the properties of the traditional blade have been optimized by a suitable choice of steel composition and by treatments such as annealing.
In theory, it might be possible to increase the service life of such a coating blade by using a blade material which in itself is more wear-resistant than the conventional spring-steel. Materials apparently suitable, such as hard metals and cermets, are not always sufficiently flexible. Indeed such materials are often extremely brittle and would therefore break easily due to the stresses normally occurring from time to time in use of a doctor blade.
In technical fields other than coating, attempts have previously been made to solve wear problems by attaching pieces or strips of more wear-resistant material to the carrier material used. Hard chromium plating or plating with some other metal has also been suggested as a means of giving inherently soft carrier materials a better wearing surface. Extensive experiments carried out to solve the problem of wear in doctor blades in similar ways have been unsuccessful with the thin coating blades used in paper manufacture. These known solutions proved difficult to implement from the technical point of view with the thin blade material used and it was also found that the desirable properties of the thin basic blade material, such as flexibility, necessary for good coating results, were considerably detracted from by the measures adopted to improve wear-resistance. It is, of course, essential that a coating blade having an improved wear-resistance as compared with conventional blades should have not only a longer service life, but still give a perfectly satisfactory coating result, if it is to be adopted.
Such a result is not achieved if the flexibility and uniform surface of the blade decreases as a result of the measures adopted to increase wear-resistance.